What's the best way to clean these? They get really dusty inside and the art doesn't show up but dusting them just seems to smear it around. Is it ok to clean the cardboard and gel with a slightly damp cloth? Or will that damage them?
The gel will be fine with a damp wipedown.
The cardboard... be very careful. I wiped down my BZ cardboard with a very-very-lightly damp rag, and it wasn't damaged.
BTW, you may get more feedback in the "cosmetic" sub-forum.
I did the same with the inner portion of my DOT. A very slightly dampened microfiber towel, albeit the microfiber towel Aline might pick up a fair amount of dirt/dust.